Fair Value Measurements (Assets and Liabilities Measured at Fair Value Recurring) (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Transfers of Liabilities, Level 1 to Level 2	$ 0	$ 0
Transfers of Assets, Level 1 to Level 2	0	0
Transfers of Liabilities, Level 2 to Level 1	0	0
Transfers of Assets, Level 2 to Level 1	0	0
Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	433,200,000	340,100,000
Investments at fair value	1,626,800,000	1,812,500,000
Embedded contractual derivative	7,200,000	11,200,000
State-sponsored health plans settlement account deficit	62,900,000
Total assets at fair value	2,130,100,000	2,163,800,000
Recurring [Member] | Residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	203,900,000	272,400,000
Recurring [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	144,800,000	223,100,000
Recurring [Member] | Other asset-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	41,800,000	23,600,000
Recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	23,700,000	25,900,000
Recurring [Member] | U.S. Agency securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Obligations of states and other political subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	757,000,000	842,100,000
Recurring [Member] | Corporate debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	455,600,000	425,400,000
Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	433,200,000	340,100,000
Investments at fair value	23,700,000	25,900,000
Embedded contractual derivative	0	0
State-sponsored health plans settlement account deficit	0
Total assets at fair value	456,900,000	366,000,000
Recurring [Member] | Level 1 [Member] | Residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 1 [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 1 [Member] | Other asset-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 1 [Member] | U.S. Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	23,700,000	25,900,000
Recurring [Member] | Level 1 [Member] | U.S. Agency securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 1 [Member] | Obligations of states and other political subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 1 [Member] | Corporate debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 2 [Member] | Current [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Investments at fair value	1,543,300,000	1,786,400,000
Embedded contractual derivative	0	0
State-sponsored health plans settlement account deficit	0
Total assets at fair value	1,543,300,000	1,786,400,000
Recurring [Member] | Level 2 [Member] | Current [Member] | Residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	203,500,000	272,400,000
Recurring [Member] | Level 2 [Member] | Current [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	144,100,000	223,100,000
Recurring [Member] | Level 2 [Member] | Current [Member] | Other asset-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	41,800,000	23,600,000
Recurring [Member] | Level 2 [Member] | Current [Member] | U.S. Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 2 [Member] | Current [Member] | U.S. Agency securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 2 [Member] | Current [Member] | Obligations of states and other political subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	709,900,000	841,900,000
Recurring [Member] | Level 2 [Member] | Current [Member] | Corporate debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	444,000,000	425,400,000
Recurring [Member] | Level 2 [Member] | Noncurrent [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Investments at fair value	59,800,000	0
Embedded contractual derivative	0	0
State-sponsored health plans settlement account deficit	0
Total assets at fair value	59,800,000	0
Recurring [Member] | Level 2 [Member] | Noncurrent [Member] | Residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	400,000	0
Recurring [Member] | Level 2 [Member] | Noncurrent [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	700,000	0
Recurring [Member] | Level 2 [Member] | Noncurrent [Member] | Other asset-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 2 [Member] | Noncurrent [Member] | U.S. Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 2 [Member] | Noncurrent [Member] | U.S. Agency securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 2 [Member] | Noncurrent [Member] | Obligations of states and other political subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	47,100,000	0
Recurring [Member] | Level 2 [Member] | Noncurrent [Member] | Corporate debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	11,600,000	0
Recurring [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Investments at fair value	0	200,000
Embedded contractual derivative	7,200,000	11,200,000
State-sponsored health plans settlement account deficit	62,900,000
Total assets at fair value	70,100,000	11,400,000
Embedded contractual derivative	0	3,200,000
Total liability at fair value	0	3,200,000
Recurring [Member] | Level 3 [Member] | Residential mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 3 [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 3 [Member] | Other asset-backed securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 3 [Member] | U.S. Treasury securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 3 [Member] | U.S. Agency securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	0
Recurring [Member] | Level 3 [Member] | Obligations of states and other political subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	0	200,000
Recurring [Member] | Level 3 [Member] | Corporate debt securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	$ 0	$ 0